Fix Travis-CI

[jbtule]

No description provided.

[BryanAvery]

What is the problem with this and what is required to be done?

[jbtule]

Travis-CI builds and run unit tests on pull-requests. Right now it's failing, but not because of an issue with the code, but because of the build config https://travis-ci.org/jbtule/keyczar-dotnet. The current config isn't working, says mono doesn't exist. This is likely because it uses an old config not meant technically meant for c# and something has changes on travis-ci. However, there is documentation now for an official csharp language config. https://docs.travis-ci.com/user/languages/csharp/ and so the .travis.yml needs to be updated to fit with this new config.